History of Flight
On June 28, 2015, at 1745 eastern daylight time, a Beech A36, registration N5626D, impacted a residence and terrain after a total loss of engine power near Plainville, Massachusetts. The commercial pilot and two passengers sustained fatal injuries, and the airplane was consumed by postcrash fire. The flight was conducted under 14 Code of Federal Regulations Part 91 as a personal flight. Instrument meteorological conditions existed near the accident site at the time, and an instrument flight rules flight plan had been filed. The flight departed Lancaster Airport, Lancaster, Pennsylvania, around 1612, and was destined for Norwood Memorial Airport (OWD) in Norwood, Massachusetts.
Review of radar and voice communication data from the Federal Aviation Administration (FAA) showed that the pilot was preparing for an instrument approach to OWD. When the flight was about 15 miles from OWD at 3,300 feet mean sea level (msl), the pilot declared an emergency, reporting an “engine problem.” The controller provided vectors to an alternate airport. About 30 seconds later, the pilot reported inability to maintain altitude and described a “real bad vibration” and losing engine power. The controller suggested a highway about 2.5 miles away, but the pilot responded that they had no engine and were in instrument meteorological conditions (IMC). The pilot acknowledged vectors toward the highway and stated “we’re gliding.” The last radar return showed the airplane at 700 feet msl and 66 knots groundspeed, about 0.1 mile from the accident site.
Several witnesses reported hearing engine noise followed by silence; they could not see the airplane due to low clouds. One witness described a “low moan buzzing sound” and saw the airplane wobbling over trees before a “boom.” Another witness heard a sound like a “broken fan” before impact.
Personnel and Aircraft Information
The pilot held a commercial pilot certificate with single-engine land, multiengine land (limited center thrust), and instrument ratings. The pilot’s logbook was fire-damaged and illegible. The most recent FAA third-class medical certificate was issued April 17, 2014, with 1,500 total flight hours reported.
The six-seat, low-wing airplane was manufactured in 1990, powered by a Continental IO-550-B6A 300-horsepower six-cylinder engine driving a three-bladed propeller. The most recent 100-hour inspection occurred June 5, 2015, at 2,143 total hours; the engine had been overhauled in June 2011 and had accumulated 410 hours since overhaul.
Engine maintenance logs showed no metal or debris found during oil filter examinations at the last inspection and in five of seven oil changes since overhaul. Oil analysis was normal. At overhaul, an overhauled primary alternator and a new parts manufacturer approval (PMA) alternator drive coupling were installed. The alternator was replaced twice in October 2011. No logbook entries specifically mentioned the alternator drive coupling. The coupling had no life limit, but the manufacturer recommended replacement at engine overhaul. The airplane also had a standby alternator under a supplemental type certificate.
Meteorological Information
Weather observation at North Central State Airport (SFZ) at 1735, about 8 miles southwest of the accident site, reported an overcast ceiling at 800 feet above ground level, wind 010° at 9 knots, visibility 9 statute miles, temperature 14°C, dew point 11°C, and altimeter 29.70 inches of mercury.
Wreckage and Impact Information
The airplane struck the corner of a residence’s roof and backyard deck, coming to rest upright about 15 feet from the rear wall, oriented on a magnetic heading of 320°. The airplane and most of the residence were consumed by postcrash fire.
Flight control continuity was established; left rudder and aileron cables showed overload fractures. The right flap actuator indicated the flap was retracted; the left flap actuator was consumed. Landing gear and actuators were extended. First responders noted a strong fuel odor.
Engine examination revealed a breached crankcase over the No. 6 cylinder barrel, two puncture holes near the No. 1 connecting rod, and a fractured oil sump. Bearing material, connecting rods, lifters, and crankcase fragments were found in the sump and outside the crankcase.
The alternator drive coupling was loose on the alternator shaft, with the housing separated into two pieces. Abnormal wear was present on the shaft collar and gear teeth. The thrust washer showed abnormal wear; the woodruff key was missing.
The face gear that drove the coupling was separated from its crankshaft flange; all four attachment bolts and locking tab remnants were found in the oil sump. The crankcase adjacent to the face gear was damaged. Metal particles, piston and connecting rod fragments, and orange elastomer particles (consistent with coupling material) were found throughout the engine. Four connecting rods were separated from the crankshaft; three showed thermal discoloration and mechanical damage consistent with lack of lubrication. The two remaining rods (Nos. 3 and 4) would not rotate, and their bearings displayed thermal distress. Oil passages were mostly clear except No. 6 was partially obstructed.
Additional Information
Coupling Inspection and Maintenance Procedures
The engine manufacturer’s maintenance manual required examination of the coupling and face gear during the 500-hour inspection, including a “slip test.” The OEM coupling could be reused indefinitely if it passed inspection. The PMA coupling manufacturer recommended replacement at major overhaul.
Alternator Failure Indication System
The airplane had a “LOW BUS VOLTS” annunciator for alternator failure. A coupling failure could also trigger this light, but there was no way to differentiate mechanical coupling failure from electrical alternator failure.
Alternator Failure Emergency Procedures
The Beech A36 pilot’s operating handbook specified that the final step for alternator failure (with or without standby alternator) was to “land as soon as practical.”
Tests and Research
NTSB materials laboratory testing revealed two conditions that could cause coupling failure: 1. Improper assembly torque (incorrect values, lack of lubrication, or inadequate instructions) could lead to progressive loosening and abnormal wear. Multiple installation instructions existed with varying detail; only the engine manufacturer’s instructions specified lubricating threads. 2. An out-of-tolerance coupling assembly (gear not fully seated) could cause insufficient clamping force, also leading to loosening. This condition was observed in exemplar couplings that had undergone improper slip testing. No published procedures exist to inspect for this condition.
Over a 10-year period, an engine overhaul facility observed several couplings with failures or abnormal wear, including worn washers, damaged keys, and elastomer disintegration. The facility notified the FAA upon discovering failures. Most occurred in twin-engine aircraft with reverse-rotation engines. FAA service difficulty reports contained 10 possibly related entries, but with limited detail. Engine manufacturer warranty records included six claims of coupling failure or damage.
Medical and Pathological Information
An autopsy of the pilot determined the manner of death as “accident.” Toxicological testing was negative for carbon monoxide, ethanol, and drugs of abuse; ranitidine (an antihistamine) was detected but posed no flight safety hazard.